At least 4 ways to avoid this confusion:
<UL>
  <LI>Return a <EM>copy</EM>. L1 and L2 are unaffected by JOIN. Memory
      hungry.<P>
  <LI>JOIN(L1,L2) transfers elements from L2 to L1. L2 ends up empty.<P>
  <LI>Change `type' of L2 from List to Sublist. Restrict some
      operations to operate only on Lists.<P>
  <LI>L1 and L2 become exactly the same list: all subsequent
      operations on one of them have identical effects on both.
</UL>
This is a crucial consideration in formulating our specification.

<P>We choose the following variation: JOIN(L1,L2) transfers elements
from L2 to L1, but L2 ends up <EM>undefined</EM> rather than merely
<EM>empty</EM>.